U.S. patent application number 13/893987 was filed with the patent office on 2014-11-20 for star tracker with steerable field-of-view baffle coupled to wide field-of-view camera.
The applicant listed for this patent is The Charles Stark Draper Laboratory, Inc.. Invention is credited to Murali Chaparala, Robin Mark Adrian Dawson, Juha Pekka Laine.
Application Number | 20140340522 13/893987 |
Document ID | / |
Family ID | 51895475 |
Filed Date | 2014-11-20 |
United States Patent
Application |
20140340522 |
Kind Code |
A1 |
Dawson; Robin Mark Adrian ;
et al. |
November 20, 2014 |
STAR TRACKER WITH STEERABLE FIELD-OF-VIEW BAFFLE COUPLED TO WIDE
FIELD-OF-VIEW CAMERA
Abstract
A star tracker has an electronically steerable point of view,
without requiring a precision aiming mechanism. The star tracker
can be strapped down, thereby avoiding problems associated with
precision aiming of mechanical devices. The star tracker images
selectable narrow portions of a scene, such as the sky. Each
stellar sighting can image a different portion of the sky,
depending on which navigational star or group of navigational stars
is of interest. The selectability of the portion of the sky imaged
enables the star tracker to avoid unwanted light, such as from the
sun.
Inventors: |
Dawson; Robin Mark Adrian;
(Watertown, MA) ; Laine; Juha Pekka; (Boston,
MA) ; Chaparala; Murali; (Newton, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Charles Stark Draper Laboratory, Inc. |
Cambridge |
MA |
US |
|
|
Family ID: |
51895475 |
Appl. No.: |
13/893987 |
Filed: |
May 14, 2013 |
Current U.S.
Class: |
348/169 |
Current CPC
Class: |
H04N 5/2259 20130101;
B64G 1/361 20130101 |
Class at
Publication: |
348/169 |
International
Class: |
H04N 5/225 20060101
H04N005/225 |
Claims
1. A star tracker, comprising: a camera having a field of view; and
an electronically adjustable baffle assembly disposed relative to
the camera and configured to expose a selectable portion, less than
all, of the camera field of view to a scene.
2. A star tracker according to claim 1, wherein the selectable
portion of the camera field of view is circular.
3. A star tracker according to claim 1, wherein the camera field of
view is greater than about 10.degree..
4. A star tracker according to claim 1, wherein the selectable
portion of the camera field of view comprises less than about 30%
of the camera field of view.
5. A star tracker according to claim 1, wherein the baffle assembly
comprises at least a portion of a dome, the dome defining an
aperture configured to define the selectable portion of the camera
field of view exposed to the scene, the baffle assembly being
rotatable about an optical axis of the camera.
6. A star tracker according to claim 1, wherein the baffle assembly
comprises at least a portion of a dome, the dome defining an
aperture configured to expose the selectable portion of the camera
field of view to the scene, the baffle assembly being rotatable
about an optical axis of the camera.
7. A star tracker according to claim 6, wherein the aperture is
positionable along an arc that intersects, and is coplanar with,
the optical axis of the camera.
8. A star tracker according to claim 6, wherein the aperture is
positionable within the camera field of view.
9. A star tracker according to claim 7, wherein the baffle assembly
comprises a baffle having an axis that coincides with an optical
axis of the selectable portion of the camera field of view.
10. A star tracker according to claim 1, wherein the selectable
portion of the field of view of the camera comprises at least two
discontiguous regions of the field of view of the camera.
11. A star tracker according to claim 1, wherein the baffle
assembly comprises a plurality of elements, wherein transparency of
each element of the plurality of elements is electronically
controllable, such that the selectable portion of the field of view
of the camera is exposed to the scene through at least one
transparent element of the plurality of elements and a remaining
portion of the field of view of the camera is obscured from the
scene by at least one non-transparent element of the plurality of
the elements.
12. A star tracker according to claim 1, wherein size of the
selectable portion of the field of view of the camera is
electronically adjustable.
13. A star tracker according to claim 1, wherein the camera
comprises a monocentric objective lens.
14. A star tracker according to claim 13, wherein the camera
comprises a plurality of pixelated image sensor arrays and a
plurality of optical fibers optically coupling each pixelated image
sensor array of the plurality of pixelated image sensor arrays to
the monocentric objective lens.
15. A star tracker according to claim 1, further comprising: a
first rate sensor having a first sensory axis and being
mechanically coupled to the camera; a second rate sensor having a
second sensory axis perpendicular to the first sensory axis and
being mechanically coupled to the camera; and a controller coupled
to the camera, the baffle, the first rate sensor and the second
rate sensor and configured to: measure vibration of the camera,
based on input signals from the first rate sensor and the second
rate sensor; and process an image captured by the camera, based on
the vibration.
16. A star tracker according to claim 1, further comprising a
controller coupled to the camera and the baffle assembly and
configured to: cause the camera to capture a first image; then
adjust the baffle assembly, such that a different portion of the
camera field of view is exposed to the scene; and then cause the
camera to capture a second image.
17. A star tracker according to claim 16, wherein the controller is
configured to determine a location of the camera, based at least in
part on an analysis of at least a portion of the first image and at
least a portion of the second image.
18. A star tracker according to claim 1, further comprising a
controller coupled to the camera and the baffle assembly and
configured to: adjust the baffle assembly, such that the selectable
portion of the camera field of view includes a portion of the scene
expected to include a space object having a predictable location;
cause the camera to capture an image; and determine a location of
the camera, based at least in part on information about the space
object and an analysis of at least a portion of the image.
19. A star tracker according to claim 18, wherein the space object
comprises an astronomical object.
20. A star tracker according to claim 18, wherein the space object
comprises an artificial satellite.
21. A star tracker according to claim 18, wherein the controller is
configured to determine the location of the camera based at least
in part on dispersion of light from the space object through
earth's atmospheric limb.
22. A star tracker according to claim 18, wherein the controller is
configured to determine the location of the camera based at least
in part on refraction of light from the space object through
earth's atmospheric limb.
23. A star tracker according to claim 1, further comprising a
controller coupled to the camera and the baffle assembly and
configured to: cause the camera to capture an image; and analyze a
portion, less than all, of the image, the portion of the image
corresponding to the portion of the camera field of view exposed to
the scene.
24. A star tracker according to claim 1, wherein: the camera
comprises a plurality of image sensor arrays, each image sensor
array of the plurality of image sensor arrays comprising a
plurality of pixels; and further comprising: a controller coupled
to the camera and the baffle assembly and configured to read a
subset, less than all, of the pixels of the plurality of image
sensor arrays, the subset corresponding to the selectable portion
of the camera field of view exposed to the scene.
25. A method for exposing a selectable portion, less than all, of a
field of view of a camera to a scene, the method comprising:
disposing a baffle assembly adjacent the camera, such that the
camera is aimed toward an interior of the baffle assembly, the
baffle assembly being configured to define an aperture whose
position on the baffle assembly is electronically adjustable and
such that the aperture defines the selectable portion, less than
all, of the field of view of the camera exposed to the scene; and
under control of a processor, adjusting the position of the
aperture on the baffle assembly, such that the aperture is oriented
toward the scene.
26. A method according to claim 25, wherein: the baffle assembly
comprises a dome that defines an elongated opening extending along
a longitude of the dome; the method further comprising: disposing a
curtain within the opening, the curtain being movable along the
longitude of the dome, the curtain obscuring the opening from the
camera field of view, except the curtain defining the aperture;
wherein adjusting the position of the aperture comprises: under
control of a processor, rotating the dome about an axis of symmetry
of the dome, such that the opening in the dome is oriented toward
the scene; and under control of a processor, moving the curtain
along the longitude of the dome, such that the aperture is oriented
toward the scene.
27. A method according to claim 25, wherein: the baffle assembly
comprises a dome comprising a plurality of elements, wherein
transparency of each element of the plurality of elements is
electronically controllable; and adjusting the position of the
aperture on the baffle assembly comprises, under control of a
processor, setting transparency of at least one selected element of
the plurality of elements, such that the selectable portion of the
field of view of the camera is exposed to the scene through at
least one transparent element of the plurality of elements and a
remaining portion of the field of view of the camera is obscured
from the scene by at least one non-transparent element of the
plurality of the elements.
28. A method according to claim 27, wherein adjusting the position
of the aperture on the baffle assembly comprises, under control of
the processor, setting transparency of the at least one selected
element of the plurality of elements to adjust size of the
aperture.
29. A method according to claim 25, further comprising: under
control of a processor, measuring vibration of the camera, based on
input signals from a first rate sensor and a second rate sensor;
and processing an image captured by the camera, based on the
vibration.
30. A method according to claim 25, further comprising: after
adjusting the position of the aperture, under control of a
processor, capturing a first image by the camera; then adjusting
the position of the aperture on the baffle assembly, such that a
different portion of the camera field of view is exposed to the
scene; and then under control of the processor, capturing a second
image by the camera.
31. A method according to claim 30, further comprising determining
a location of the camera, based at least in part on an analysis of
at least a portion of the first image and at least a portion of the
second image.
32. A method according to claim 25, wherein: adjusting the position
of the aperture comprises automatically adjusting the position of
the aperture such that the selectable portion of the camera field
of view includes a portion of the scene expected to include a space
object having a predictable location; causing the camera to capture
an image; and automatically determining a location of the camera,
based at least in part on information about the space object and an
analysis of at least a portion of the image.
33. A method according to claim 32, wherein the space object
comprises an astronomical object.
34. A method according to claim 32, wherein the space object
comprises an artificial satellite.
35. A method according to claim 32, wherein determining the
location of the camera comprises determining the location of the
camera based at least in part on dispersion of light from the space
object through earth's atmospheric limb.
36. A method according to claim 32, wherein determining the
location of the camera comprises determining the location of the
camera based at least in part on refraction of light from the space
object through earth's atmospheric limb.
37. A method according to claim 25, further comprising:
automatically causing the camera to capture an image; and
automatically analyzing a portion, less than all, of the image, the
portion of the image corresponding to the portion of the camera
field of view exposed to the scene.
38. A method according to claim 25, wherein: the camera comprises a
plurality of image sensor arrays, each image sensor array of the
plurality of image sensor arrays comprising a plurality of pixels;
the method further comprising: reading a subset, less than all, of
the pixels of the plurality of image sensor arrays, the subset
corresponding to the selectable portion of the camera field of view
exposed to the scene.
39. A computer program product for exposing a selectable portion,
less than all, of a field of view of a camera to a scene, wherein a
baffle assembly is disposed adjacent the camera, such that the
camera is aimed toward an interior of the baffle assembly, the
baffle assembly being configured to define an aperture whose
position on the baffle assembly is electronically adjustable and
such that the aperture defines the selectable portion, less than
all, of the field of view of the camera exposed to the scene, the
computer program product comprising a non-transitory
computer-readable medium having computer readable program code
stored thereon, the computer readable program code being configured
to cause the processor to perform operations including: adjusting
the position of the aperture on the baffle assembly, such that the
aperture is oriented toward the scene.
40. A computer program product according to claim 39, wherein the
baffle assembly comprises a dome that defines an elongated opening
extending along a longitude of the dome, a curtain is disposed
within the opening, the curtain being movable along the longitude
of the dome, the curtain obscuring the opening from the camera
field of view, except the curtain defining the aperture and the
computer readable program code is configured to adjust the position
of the aperture by causing the processor to perform operations
including: rotating the dome about an axis of symmetry of the dome,
such that the opening in the dome is oriented toward the scene; and
moving the curtain along the longitude of the dome, such that the
aperture is oriented toward the scene.
41. A computer program product according to claim 39, wherein the
baffle assembly comprises a dome comprising a plurality of
elements, wherein transparency of each element of the plurality of
elements is electronically controllable, and the computer readable
program code is configured to adjust the position of the aperture
by causing the processor to perform an operation including: setting
transparency of at least one selected element of the plurality of
elements, such that the selectable portion of the field of view of
the camera is exposed to the scene through at least one transparent
element of the plurality of elements and a remaining portion of the
field of view of the camera is obscured from the scene by at least
one non-transparent element of the plurality of the elements.
Description
TECHNICAL FIELD
[0001] The present invention relates to star trackers and, more
particularly, to strap-down, wide field-of-view (FOV) start
trackers that include steerable field-of-view baffles.
BACKGROUND ART
[0002] Most artificial satellites, spacecraft and other propelled
devices such as aircraft, ship and ground vehicles (collectively
referred to herein as vehicles) require information about their
locations and/or attitudes to accomplish their missions. This
information may be obtained from one or more sources, such as the
global positioning system (GPS), ground-based radar tracking
stations and/or an on-board star tracker.
[0003] A star tracker is an optical device that measures bearing(s)
to one or more stars, as viewed from a vehicle. A star tracker
typically includes a star catalog that lists bright navigational
stars and information about their locations in the sky, sufficient
to calculate a location of a vehicle in space, given bearings to
several of the stars. A conventional star tracker includes a lens
that projects an image of a star onto a photocell, or that projects
an image of one or more stars onto a light-sensitive sensor array
(digital camera).
[0004] One type of star tracker is "strapped-down," meaning its
view angle, relative to its vehicle, is fixed. Another type of star
tracker can be aimed mechanically, such as in a direction in which
a navigational star is expected to be seen. Using data from the
photocell or sensor array, the star catalog and information about
the star tracker's view angle, relative to the vehicle, the star
tracker calculates a position of the vehicle in space.
[0005] Strapped-down star trackers are mechanically simpler than
mechanically aimable star trackers. However, the fixed view angle
of a strapped-down star tracker limits the number of navigational
stars that may be used. Mechanically aimable start trackers can use
a larger number of navigational stars. However, aiming a prior art
star tracker, relative to its vehicle, with the required precision
poses substantial problems. In either case, preventing stray light,
such as from the sun or reflected from the moon, reaching the
photocell or sensor array is challenging, particularly when a
navigational star of interest is apparently close to one of these
very bright objects.
SUMMARY OF EMBODIMENTS
[0006] An embodiment of the present invention provides a star
tracker. The star tracker includes a camera and an electronically
adjustable baffle assembly. The camera has a field of view. The
electronically adjustable baffle assembly is disposed relative to
the camera. The electronically adjustable baffle assembly is
configured to expose a selectable portion, less than all, of the
camera field of view to a scene.
[0007] The selectable portion of the camera field of view may be
circular. The camera field of view may be greater than about
10.degree.. The selectable portion of the camera field of view may
include less than about 30% of the camera field of view.
[0008] The baffle assembly may include at least a portion of a
dome. The dome may define an aperture. The aperture may be
configured to define the selectable portion of the camera field of
view exposed to the scene. The baffle assembly may be rotatable
about an optical axis of the camera.
[0009] The baffle assembly may include at least a portion of a
dome. The dome may define an aperture. The aperture may be
configured to expose the selectable portion of the camera field of
view to the scene. The baffle assembly may be rotatable about an
optical axis of the camera.
[0010] The aperture may be positionable along an arc that
intersects, and is coplanar with, the optical axis of the
camera.
[0011] The aperture may be positionable within the camera field of
view.
[0012] The baffle assembly may include a baffle having an axis that
coincides with an optical axis of the selectable portion of the
camera field of view.
[0013] The selectable portion of the field of view of the camera
may include at least two discontiguous regions of the field of view
of the camera.
[0014] The baffle assembly may include a plurality of elements.
Transparency of each element of the plurality of elements may be
electronically controllable. The selectable portion of the field of
view of the camera may be exposed to the scene through at least one
transparent element of the plurality of elements. Remaining portion
of the field of view of the camera may be obscured from the scene
by at least one non-transparent element of the plurality of the
elements.
[0015] Size of the selectable portion of the field of view of the
camera may be electronically adjustable.
[0016] The camera may include a monocentric objective lens.
[0017] The camera may include a plurality of pixelated image sensor
arrays and a plurality of optical fibers. The plurality of optical
fibers may optically couple each pixelated image sensor array of
the plurality of pixelated image sensor arrays to the monocentric
objective lens.
[0018] The star tracker may also include a first rate sensor, a
second rate sensor and a controller. The first rate sensor may have
a first sensory axis. The first rate sensor may be mechanically
coupled to the camera. The second rate sensor may have a second
sensory axis perpendicular to the first sensory axis. The second
rate sensor may be mechanically coupled to the camera. The
controller may be coupled to the camera, the baffle, the first rate
sensor and the second rate sensor. The controller may be configured
to measure vibration of the camera, based on input signals from the
first rate sensor and the second rate sensor. The controller may be
further configured to process an image captured by the camera,
based on the vibration.
[0019] The star tracker may also include a controller coupled to
the camera and the baffle assembly. The controller may be
configured to cause the camera to capture a first image. The
controller may be configured to then adjust the baffle assembly,
such that a different portion of the camera field of view is
exposed to the scene. The controller may be configured to then
cause the camera to capture a second image.
[0020] The controller may be configured to determine a location of
the camera, based at least in part on an analysis of at least a
portion of the first image and at least a portion of the second
image.
[0021] The star tracker may also include a controller coupled to
the camera and the baffle assembly. The controller may be
configured to adjust the baffle assembly, such that the selectable
portion of the camera field of view includes a portion of the scene
expected to include a space object having a predictable location.
The controller may be further configured to cause the camera to
capture an image and determine a location of the camera, based at
least in part on information about the space object and an analysis
of at least a portion of the image.
[0022] The space object may be or include an astronomical object
and/or an artificial satellite.
[0023] The controller may be configured to determine the location
of the camera based at least in part on dispersion and/or
refraction of light from the space object through earth's
atmospheric limb.
[0024] The star tracker may include a controller coupled to the
camera and the baffle assembly. The controller may be configured to
cause the camera to capture an image and analyze a portion, less
than all, of the image. The portion of the image may correspond to
the portion of the camera field of view exposed to the scene.
[0025] The camera may include a plurality of image sensor arrays.
Each image sensor array of the plurality of image sensor arrays may
include a plurality of pixels. The star tracker may also include a
controller coupled to the camera and the baffle assembly. The
controller may be configured to read a subset, less than all, of
the pixels of the plurality of image sensor arrays. The subset may
correspond to the selectable portion of the camera field of view
exposed to the scene.
[0026] Another embodiment of the present invention provides a
method for exposing a selectable portion, less than all, of a field
of view of a camera to a scene. The method includes disposing a
baffle assembly adjacent the camera. The camera is aimed toward an
interior of the baffle assembly. The baffle assembly is configured
to define an aperture whose position on the baffle assembly is
electronically adjustable. The aperture defines the selectable
portion, less than all, of the field of view of the camera exposed
to the scene. Under control of a processor, the position of the
aperture on the baffle assembly is adjusted, such that the aperture
is oriented toward the scene.
[0027] The baffle assembly may include a dome that defines an
elongated opening extending along a longitude of the dome. The
method may include disposing a curtain within the opening. The
curtain may be movable along the longitude of the dome. The curtain
may obscure the opening from the camera field of view, except the
portion of the curtain defining the aperture. Adjusting the
position of the aperture may include, under control of a processor,
rotating the dome about an axis of symmetry of the dome, such that
the opening in the dome is oriented toward the scene. Adjusting the
position of the aperture may also include, under control of a
processor, moving the curtain along the longitude of the dome, such
that the aperture is oriented toward the scene.
[0028] The baffle assembly may include a dome that includes a
plurality of elements. Transparency of each element of the
plurality of elements may be electronically controllable. Adjusting
the position of the aperture on the baffle assembly may include,
under control of a processor, setting transparency of at least one
selected element of the plurality of elements, such that the
selectable portion of the field of view of the camera is exposed to
the scene through at least one transparent element of the plurality
of elements. A remaining portion of the field of view of the camera
may be obscured from the scene by at least one non-transparent
element of the plurality of the elements.
[0029] Adjusting the position of the aperture on the baffle
assembly may include, under control of the processor, setting
transparency of the at least one selected element of the plurality
of elements to adjust size of the aperture.
[0030] Optionally, under control of a processor, vibration of the
camera may be measured, based on input signals from a first rate
sensor and a second rate sensor. An image captured by the camera
may be processed, based on the vibration.
[0031] After adjusting the position of the aperture, under control
of a processor, a first image may be captured by the camera. Then,
the position of the aperture on the baffle assembly may be
adjusted, such that a different portion of the camera field of view
is exposed to the scene. Then, under control of the processor, a
second image may be captured by the camera.
[0032] Optionally, a location of the camera may be determined,
based at least in part on an analysis of at least a portion of the
first image and at least a portion of the second image.
[0033] Adjusting the position of the aperture may include
automatically adjusting the position of the aperture such that the
selectable portion of the camera field of view includes a portion
of the scene expected to include a space object having a
predictable location. The camera may be caused to capture an image.
A location of the camera may be automatically determined, based at
least in part on information about the space object and an analysis
of at least a portion of the image.
[0034] The space object may be or include an astronomical object
and/or an artificial satellite.
[0035] Determining the location of the camera may include
determining the location of the camera based at least in part on
dispersion and/or refraction of light from the space object through
earth's atmospheric limb.
[0036] The camera may be automatically caused to capture an image.
A portion, less than all, of the image may be automatically
analyzed. The portion of the image that is analyzed corresponds to
the portion of the camera field of view exposed to the scene.
[0037] The camera may include a plurality of image sensor arrays.
Each image sensor array of the plurality of image sensor arrays may
include a plurality of pixels. The method may further include
reading a subset, less than all, of the pixels of the plurality of
image sensor arrays. The subset may correspond to the selectable
portion of the camera field of view exposed to the scene.
[0038] Yet another embodiment of the present invention provides a
computer program product for exposing a selectable portion, less
than all, of a field of view of a camera to a scene. A baffle
assembly is disposed adjacent the camera. The camera is aimed
toward an interior of the baffle assembly. The baffle assembly is
configured to define an aperture whose position on the baffle
assembly is electronically adjustable. The aperture defines the
selectable portion, less than all, of the field of view of the
camera exposed to the scene. The computer program product includes
a non-transitory computer-readable medium. Computer readable
program code is stored on the medium. The computer readable program
code is configured to cause the processor to perform an operation,
including adjusting the position of the aperture on the baffle
assembly, such that the aperture is oriented toward the scene.
[0039] The baffle assembly may include a dome. The dome may define
an elongated opening extending along a longitude of the dome. A
curtain may be disposed within the opening. The curtain may be
movable along the longitude of the dome. The curtain may obscure
the opening from the camera field of view, except where the curtain
defines the aperture. The computer readable program code may be
configured to adjust the position of the aperture by causing the
processor to perform operations including rotating the dome about
an axis of symmetry of the dome, such that the opening in the dome
is oriented toward the scene. In addition, the curtain may be moved
along the longitude of the dome, such that the aperture is oriented
toward the scene.
[0040] The baffle assembly may include a dome. The dome may include
a plurality of elements. Transparency of each element of the
plurality of elements may be electronically controllable. The
computer readable program code may be configured to adjust the
position of the aperture by causing the processor to perform an
operation including setting transparency of at least one selected
element of the plurality of elements, such that the selectable
portion of the field of view of the camera is exposed to the scene
through at least one transparent element of the plurality of
elements. A remaining portion of the field of view of the camera
may be obscured from the scene by at least one non-transparent
element of the plurality of the elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] The invention will be more fully understood by referring to
the following Detailed Description of Specific Embodiments in
conjunction with the Drawings, of which:
[0042] FIG. 1 is a perspective schematic view of a star tracker,
according to an embodiment of the present invention.
[0043] FIG. 2 is a perspective schematic view of the star tracker
of FIG. 1, with addition of a honeycomb baffle, according to an
embodiment of the present invention.
[0044] FIG. 3 is a side schematic view of the star tracker of FIG.
1.
[0045] FIG. 4 is a top schematic view of a dome of the star tracker
of FIG. 1, according to an embodiment of the present invention.
[0046] FIG. 5 is a front schematic view of the dome of FIG. 4.
[0047] FIG. 6 is a perspective schematic view of a curtain of the
star tracker of FIG. 1, according to an embodiment of the present
invention.
[0048] FIG. 7 is a cross-sectional view of the dome of FIG. 4.
[0049] FIG. 8 is a side schematic cut-away view of the star tracker
of FIG. 1 illustrating two embodiments for handling excess portions
of the curtain of FIG. 6.
[0050] FIG. 9 is a perspective schematic view of a wide
field-of-view camera having a spherical objective lens.
[0051] FIG. 10 is a side schematic view of the camera of FIG. 9,
including a cross-sectional view of the spherical objective
lens.
[0052] FIG. 11 is a bottom schematic view of the camera of FIG.
9.
[0053] FIG. 12 schematically illustrates a hypothetical tiling of
the camera's field of view onto a plurality of image sensors,
according to an embodiment of the present invention.
[0054] FIG. 13 is a cut-away view of the star tracker of FIG. 1
illustrating placement of the camera of FIG. 9 within a body of the
star tracker, according to an embodiment of the present
invention.
[0055] FIG. 14 is a front schematic view of an adjustable iris.
[0056] FIG. 15 is a perspective schematic view of an adjustable
telescopic baffle.
[0057] FIG. 16 is a schematic block diagram of the star tracker of
FIG. 1, according to an embodiment of the present invention.
[0058] FIG. 17 is a perspective schematic view of a star tracker
with a pixelated dome, according to an embodiment of the present
invention.
[0059] FIG. 18 is a schematic block diagram of the star tracker of
FIG. 17, according to an embodiment of the present invention.
[0060] FIG. 19 schematically illustrates a hypothetical tiling of
two simultaneous camera fields of view onto a plurality of image
sensors, according to an embodiment of the present invention.
[0061] FIG. 20 schematically illustrates refraction and dispersion
of light from a navigational star by the atmosphere of the earth,
as seen from a space vehicle, according to the prior art principles
known as stellar horizon atmospheric refraction ("SHAR") and
stellar horizon atmospheric dispersion ("SHAD").
[0062] FIG. 21 schematically illustrates starlight refracted by a
given amount defining a conceptual conical surface extending into
space and having an axis passing through the center of the earth in
the direction of a navigational star, according to the prior art
principle of stellar horizon atmospheric refraction ("SHAR").
[0063] FIG. 22 contains a flowchart illustrating operations of some
embodiments of the present invention.
[0064] FIG. 23 contains a flowchart illustrating operations that
may be performed as part of one of the operations (adjusting an
aperture) of FIG. 22, according to some embodiments of the present
invention.
[0065] FIG. 24 contains a flowchart illustrating operations that
may be performed as part of one of the operations (adjusting an
aperture) of FIG. 22, according to some other embodiments of the
present invention.
[0066] FIG. 25 contains a flowchart illustrating operations that
may be performed as part of one of the operations (adjusting an
aperture) of FIG. 22, according to some embodiments of the present
invention.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0067] As used herein, the following terms have the following
definitions, unless their contexts indicate otherwise.
[0068] A "limb" is an apparent visual edge of a celestial body as
viewed from space.
[0069] A "atmospheric limb" is a thin layer near horizon, as viewed
from space, corresponding to an atmosphere.
[0070] A "skymark" is an object in orbit with a known ephemeris
that can be used for determining location based on sighting of the
object; multiple sightings on skymarks are required for
determination of multi-dimensional location in space.
[0071] In accordance with embodiments of the present invention,
methods and apparatus are disclosed for providing and operating
star trackers that have electronically steerable points of view,
without requiring precision aiming mechanisms. Consequently, the
star trackers can be strapped down, thereby avoiding problems
associated with precision aiming of mechanical devices.
Nevertheless, the star trackers can image selectable narrow
portions of a scene, such as the sky. Each stellar sighting can
image a different portion of the sky, depending on which
navigational star or group of navigational stars is of interest.
The selectability of the portion of the sky imaged enables the star
trackers to avoid unwanted light, such as from the sun.
Advantageously, mechanisms for selecting the portion of the scene
to be imaged do not require precision aiming.
[0072] Star trackers, according to the present disclosure, may be
used without resort to GPS or ground-based tracking systems.
Therefore, these star trackers find utility in military and other
applications, such as flight navigation, ground troop location,
intercontinental ballistic missiles (ICBMs) and other weapon and
transportation systems, that must function even if the GPS is
compromised or not available.
[0073] FIG. 1 is a perspective schematic view of a star tracker
100, according to an embodiment of the present invention. The star
tracker 100 includes a body 102 that houses a camera (not visible)
and an adjustable baffle assembly 104 attached to the body 102. The
camera, preferably a wide field-of-view camera, is aimed upward,
along an axis 105 of the body 102. The baffle assembly 104 is
configured to expose a selectable portion, less than all, of the
camera's field of view to a scene, such as a portion of the
sky.
[0074] The baffle assembly 104 includes a portion of a dome 106.
The dome 106 may be hemispherical, or it may include more or less
than a hemisphere. The dome 106 is rotatably coupled to the body
102, so the dome 106 can rotate as indicated by curved arrow 108,
relative to the body 102. The dome 104 includes two side portions
110 and 112 that rotate together.
[0075] The dome 104 also includes a curtain 114 rotatably coupled
to the two side portions 110 and 112, such that the curtain can
rotate as indicated by curved arrow 116, relative to the dome 104.
Thus, in this embodiment, the curtain 114 can rotate about an axis
(not shown) perpendicular to the axis 105 about which the two side
portions 110 and 112 rotate. The curtain 114 extends at least
between the two side portions 110 and 112 to prevent light entering
the interior of the baffle assembly 104, except via an aperture 120
defined by the curtain 114. The aperture 120 exposes a selectable
portion, less than all, of the camera's field of view to a scene,
such as the sky. The aperture 120 may be open or it may be made of
a transparent material, such as glass.
[0076] In this embodiment, the aperture 120 is surrounded by a
coaxial baffle 122. The baffle 122 may be frustoconical, as shown
in FIG. 1, or it may be cylindrical or another shape. The inside
surface of the baffle 122 may include concentric circular steps (as
shown in FIG. 1) and/or a honeycomb baffle 200 (as shown in FIG. 2)
to reduce unwanted reflections of stray light. Some other
embodiments do not include the baffle 122.
[0077] FIG. 3 is a side schematic view of the star tracker 100. As
noted, the curtain 114 can rotate as indicated by arrow 116. Thus,
the baffle 122 and the aperture 120 (not visible in FIG. 3) can be
positioned along an arc 300. For example, the baffle 122 may be
positioned as shown in FIG. 3, or it may be positioned at another
location, exemplified by 122'. Returning to FIG. 1, between
rotation of the curtain 114 as indicated by arrow 116 and rotation
of the dome 106 as indicated by arrow 108, the aperture 120 can be
positioned so as to expose a selected portion of the scene, such as
the sky, to the camera, thereby providing the star tracker 100 with
a steerable point of view.
[0078] FIG. 4 is a top schematic view, and FIG. 5 is a front
schematic view, of the dome 106. FIG. 6 is a perspective schematic
view of the curtain 114. Width 600 (FIG. 6) of the curtain 114 is
greater than width 400 (FIG. 4) of a gap (opening) 401 between the
two side portions 110 and 112 of the dome 106. FIG. 7 is a
cross-sectional view of the dome 106 of FIG. 4, but also includes
the curtain 114. The curtain 114 rides in tracks 402 and 404 along
respective inside surfaces of the two side portions 110 and 112 for
mechanical support and to prevent stray light entering the baffle
assembly 104. The tracks 402 and 404 may be equipped with light
seal brushes, foam strips or the like (not shown).
[0079] As the curtain 114 moves along the tracks 402 and 404,
excess portions of the curtain 114, i.e., portions of the curtain
114 not needed to block the gap 401, extend into the body 102, as
schematically illustrated in FIG. 8. FIG. 8 is a side schematic
cut-away view of the star tracker 100 illustrating two embodiments
for handling the excess portions of the curtain 114. In one
embodiment, illustrated on the left side of FIG. 8, excess portions
of the curtain 114 are wound on a spool 800. The spool 800 may be
motor driven or spring wound. The spool 800 is mechanically coupled
to the dome 106 for rotation therewith, in the directions of arrow
108.
[0080] In the other embodiment, illustrated on the right side of
FIG. 8, excess portions of the curtain 114 extend into a pocket 802
defined by an inner wall 804 of the body 102. In yet another
embodiment (not illustrated), excess portions of the curtain 114
accordion fold into a trough defined inside the body 102 or
depending from the dome 106.
[0081] The curtain 114 may define sprocket holes 602 (FIG. 6)
adjacent its two long edges. These sprocket holes 602 may be
engaged by a sprocket gear 604 driven by a motor 606 to move the
curtain 114 along the tracks 402 and 404. Similarly, the dome 106
may include a rack gear 700 (FIG. 7) along its inside perimeter.
This rack gear 700 may be engaged by a pinion gear 702 driven by a
motor 704 to rotate the dome 106 to a desired position, relative to
the body 102 of the star tracker 100. The curtain 114 may be made
of a single flexible member, or it may include several flexible or
rigid individual members (as suggested by lines, such as line 124,
in FIG. 1) hingedly or otherwise chained together. The curtain 114,
or sections thereof, may be pulled from the body 102, and it or
they may ride in a slot to keep it or them aligned to the rest of
the hemispherical dome 104.
Camera
[0082] As noted, the star tracker 100 may include a wide
field-of-view camera within the body 102. FIG. 9 is a perspective
schematic view of an exemplary wide field-of-view camera 900 having
a spherical objective lens 902. The lens 902 is coupled via a
plurality of approximately 8.5-14 mm long optical fiber bundles,
exemplified by fiber bundles 904, 906, 908 and 910, to respective
square, rectangular or other shaped pixelated planar image sensor
arrays, exemplified by arrays 912, 914, 916 and 918. Each optical
fiber should be polished to match the spherical surface of the lens
902. The optical fibers should be subject to at most very little
physical distortion (on the order of <<1%), if the image
sensor pitch matches the fiber bundle pitch. Suitable fiber bundles
(2.4 mm pitch, N.A.=1, 1.84/1.48 core clad index) are available
from SCHOTT Corporation (SCHOTT North America, Inc., 555 Taxter
Road, Elmsford, N.Y. 10523). Thus, each image sensor array 912-918,
etc. receives light from a portion of the camera's field of
view.
[0083] FIG. 12 schematically illustrates a hypothetical tiling of
the camera's field of view 1200 onto a plurality of rectangular
image sensors, exemplified by image sensor arrays 912-918, 1204,
1206, 1208, 1210, 1212, 1214 and 1216. Returning to FIG. 9,
multi-pin connectors, such as connector 920, accept flexible
printed wiring or other suitable cables to interconnect the camera
900 to a processor or other image-processing circuitry (not shown).
Multiple high bandwidth multi-lane low-voltage differential
signaling (LVDS) data channels may be used to couple the image
sensor arrays 912-918, etc. to one or more field-programmable gate
arrays (FPGAs), and a single high bandwidth SERDES link (operating
at approximately 3.2 Gb/sec.) may couple the FPGAs to a CEV or
other processor.
[0084] Alternatively, the lens 902 may be optically coupled, via
optical fibers, a gap or another intermediary, to one or more
spherical cap-shaped sensor arrays (not shown).
[0085] As shown schematically in FIG. 10, the lens 902 may include
a plurality of monocentric shells, exemplified by shells 1000 and
1002, to correct for spherical and chromatic aberrations. (The
camera shown in FIG. 10 includes more image sensor arrays than the
camera shown in FIG. 9.) The lens 902 may include a central
approximately 4 mm diameter aperture 1004 defined by a fixed or
adjustable iris 1006. FIG. 11 is a bottom schematic view of the
camera of FIG. 10 showing a plurality of planar image sensor
arrays.
[0086] Additional information about a suitable camera is available
in "Optimization of two-glass monocentric lenses for compact
panoramic imagers: general aberration analysis and specific
designs," by Igor Stamenov, Ilya P. Agurok and Joseph E. Ford,
Applied Optics, Vol. 51, No. 31, Nov. 1, 2012, pp. 7648-7661, as
well as U.S. Pat. No. 3,166,623 titled "Spherical Lens Imaging
Device," by J. A. Waidelch, Jr., filed Dec. 29, 1960, the entire
contents of all of which are hereby incorporated by reference
herein. The camera 900 is conceptually similar to a larger
monocentric objective camera called AWARE2 and developed at Duke
University.
[0087] FIG. 13 is a cut-away view of the star tracker 100
illustrating placement of the camera 900 within the body 102. The
camera 900 optical axis 1200 aligns with the axis 105 (FIG. 1) of
the body 102. In an embodiment, the camera 900 has a 120.degree.
field of view, although cameras with other fields of view may be
used. However, the dome 106 and the curtain 114 block all of the
camera's field of view, except through the aperture 120. Thus, size
and shape of the aperture 120 and configuration (size, shape and
length) of the baffle 122 (if any), as well as rotational position
of the curtain 114 along the arc 300 (FIG. 3) and rotational
position of the dome 106, relative to the body 102, i.e., along the
direction of the arrow 108 (FIG. 1), determine which portion of the
camera's field of view is exposed ("the selectable portion of the
camera field of view") to a scene. In one embodiment the aperture
120 and the baffle 122 limit the portion of the camera's field of
view to about 3-4.degree., however in other embodiments, the
camera's field of view may be limited to larger or smaller angles,
such as about 1.degree., 10.degree. or other angles.
[0088] For example, as shown in FIG. 13, light traveling toward the
star tracker 100 along a path 1202 is passed by the aperture 120 to
the lens 902 and thence to a corresponding one or more pixels on
one or more of the image sensors 912-918. The path 1202 is referred
to herein as "an optical axis of the selectable portion of the
camera field of view." FIG. 12 illustrates a hypothetical portion
1202 of the camera's field of view that is exposed by the aperture
120 to the scene. In the example illustrated in FIG. 12, the
selectable portion of the camera field of view spans more than one
image sensor array 912, 914, 1204, 1206, 1208, 1210, 1212, 1214 and
1216. However, with other size apertures 120, other configurations
of the baffle 122 and/or other size image sensor arrays, the
selectable portion of the camera field of view may span more of
fewer image sensor arrays.
[0089] The size of the aperture 120 and the configuration of the
baffle 122 (if any) determine the size of the selectable portion of
the camera field of view. Other embodiments may include variable
apertures, such as an adjustable iris 1400 shown in FIG. 14, and/or
variable baffles, such as a telescopic baffle 1500 shown in FIG.
15. Opening or closing the adjustable iris 1400, such as by a drive
motor (not shown in FIG. 14, but discussed below), varies an amount
of the scene exposed to the camera. Extending or retracting an
inner baffle tube 1502, relative to an outer baffle tube 1504, as
indicated by arrow 1506, varies an amount of the scene exposed to
the camera. The inner and outer baffle tubes 1502 and 1504 may, in
some embodiments, be matingly threaded, such that rotating the
inner baffle tube 1502 by a motor (not shown in FIG. 15, but
discussed below), relative to the outer baffle tube 1504, extends
or retracts the inner baffle tube 1502.
Angular Rate Sensors
[0090] Some embodiments of the star tracker include mutually
perpendicular angular rate sensors 126 and 128 (FIG. 1), both
oriented perpendicular to the axis 105 of the body 102. These rate
sensors 126 and 128 may be used by a controller (described below)
to sense movement, such as vibration, of the star tracker 100 and
to compensate for this movement while analyzing images from the
sensors 912-918, etc. Such compensation may be advantageous in
cases where the star tracker 100 experiences vibrations having a
frequency greater than about 100 Hz. Such compensation allows the
camera 900 or a controller to maintain knowledge of the direction
of sightings, relative to previous sighting, to ensure accuracy of
positions that are ascertained based on multiple sightings.
Controller and Block Diagram
[0091] FIG. 16 is a schematic block diagram of an embodiment of the
present invention. A processor-driven controller 1600 is coupled to
the rate sensors 126 and 128, the sensor arrays 912-918, etc., the
dome drive motor 704 and the curtain drive motor 606 to receive
signals and/or to control operations of these items, as described
herein. For example, pixel data may be sent by the image sensors
912-918, etc. to the controller 1600, as exemplified by connections
1602, and the controller may initiate an exposure, control length
of the exposure and send other commands, such as to control which
pixels are to be read, via control signals, as exemplified by
connection 1604. A star catalog 1606 stores information about star
locations. The star catalog 1606 may be stored in a non-volatile
memory, such as a read-only memory (ROM). If the embodiment
includes an adjustable iris and/or a variable baffle, the
controller 1600 is coupled to an iris drive motor 1608 and/or a
baffle drive motor 1610, as appropriate.
[0092] The controller 1600 may include a processor configured to
execute instructions stored in a memory. Conceptually, the
processor of the controller 1600 may process data from the rate
sensors 126 and 128, or the controller may include a separate
processor or other circuit, such as one or more field programmable
gate arrays (FPGAs), to process the data from the rate sensors 126
and 128 and compensate for vibrations experienced by the star
tracker.
[0093] Although mechanical domes, curtains, baffles and irises have
been described, these items are driven by motors, which are
controlled by the controller 1600. Thus, these items are referred
to herein as being "electronically adjustable." Collectively, the
dome, curtain, baffle (if any) and iris (if any) form an adjustable
baffle assembly that is configured to expose a selectable portion
of the camera field of view to a scene, such as the sky. The
selectable portion of the camera field of view is less than the
native field of view of the camera.
Pixelated Dome
[0094] In some other embodiments, a material whose transparency or
translucency (herein collectively referred to as "transparency")
can be electronically adjusted is used in the dome to selectively
expose a portion of the camera's field of view to a scene. FIG. 17
is a perspective schematic view of one such embodiment of a star
tracker 1700 having a pixelated dome 1702 made of, or including, a
plurality of individually switchable pixels, exemplified by pixels
1704, 1706 and 1708. Square pixels 1704-1708 are shown; however,
other shape pixel may be used. The shape, size, and number of
pixels in the dome depend on minimum size and granularity in size
desired for the selectable portion of the camera field of view. The
pixels 1704-1708, etc. may be constructed using liquid crystals,
electrochromic devices, suspended particle devices, micro-blinds or
any other type of electro-optic device or material whose
transparency is electronically controllable.
[0095] FIG. 18 is a schematic block diagram of an embodiment of the
present invention that includes a pixelated dome 1702. Most
components shown in FIG. 18 are similar to corresponding components
described above, with respect to FIG. 16. However, in the
embodiment shown of FIG. 18, a controller 1800 controls
transparency of individual pixels 1704-1708, etc. of the dome 1702
via control signals 1802. The pixels that are caused to be
transparent essentially define an aperture in the dome 1702.
Consequently, a selectable portion of the field of view of the
camera is exposed to the scene through the transparent pixel(s),
and a remaining portion of the field of view of the camera is
obscured from the scene by the non-transparent pixels. FIG. 18
shows a gap between an inside surface of the pixelated dome 1702
and a surface of the lens 902. However, in some embodiments, the
pixelated dome 1702 is attached to the surface of the lens 902.
[0096] The controller 1800 can cause two or more discontiguous
groups of the pixels 1704-1708, etc. to be transparent, essentially
creating two or more apertures in the dome 1702. Thus, the dome
1702 can expose an arbitrary number of discontiguous regions of the
field of view of the camera to a scene. For example, FIG. 19
schematically illustrates a hypothetical tiling of two simultaneous
camera fields of view 1900 and 1902 onto the camera's image sensor
arrays 912-918, etc. It should be noted that the two fields of view
can, but need not, be of different sizes and/or different shapes.
Other numbers and/or shapes of fields of view may be used. Multiple
simultaneous fields of view enable the start tracker 1700 to
simultaneously image several navigational stars, while blocking
unwanted light from other stars or very bright objects, such as the
sun.
Selective Readout from Image Sensor Arrays
[0097] In some embodiments, the total number of pixels in all the
image sensor arrays 912-918, etc. exceeds 50 million. However, only
a portion of these pixels may be exposed to a scene, regardless of
whether a movable curtain-defined aperture 120 (FIG. 1) or a
pixelated dome 1702 (FIG. 17) is used, and regardless of whether
one or more simultaneous apertures are defined. In some
embodiments, after the camera captures an image, the controller
1600 or 1800 reads all pixels of only selected ones of the sensor
arrays 912-918, etc., depending on which one or more of the sensor
arrays 912-918, etc. were exposed to portions of the scene. In some
embodiments, the controller 1600 or 1800 reads only selected ones
of the pixels in the sensor arrays 912-918, etc. that were exposed
to portions of the scene.
[0098] By reading all the pixels of only a subset of the sensor
arrays 912-918, etc., or by reading only selected pixels of the
subset of the sensor arrays, image data may be read more quickly
than if all pixels of the selected sensor arrays were read or if
all pixels of all the sensor arrays were read. Time saved by not
reading all the pixels may be used to capture additional images or
to reduce time between successive images, thereby increasing
angular resolution. Furthermore, not reading all the pixels saves
electrical power, which may be limited in some vehicles.
[0099] On the other hand, some position determining algorithms
perform better when provided with data from wider fields of view,
compared to centroiding only one or a small number of stars.
However, as noted, wide fields of view correspond to large numbers
of pixels. Some embodiments use linear compressive sensing. In
these embodiments, the camera 900 or sensor arrays 912-918, etc.
compress the image data, thereby reducing the amount of data sent
to the controller 1600 or 1800, and the controller analyzes the
image data in the compressed domain. In these embodiments, the star
catalog 1606 may also be compressed. For additional information
about such compression, reference should be had to U.S. patent
application Ser. No. 12/895,004 (U.S. Pat. Publ. No. 2012/0082393)
titled "Attitude Estimation with Compressive Sampling of Starfield
Data" filed Sep. 30, 2010 by Benjamin F. Lane, et al., which is
assigned to the assignee of the present application, the entire
contents of which are hereby incorporated by reference herein.
[0100] Stellar Horizon Atmospheric Dispersion or Refraction
(SHAD/SHAR)
[0101] As noted, a star tracker measures bearing(s) to one or more
navigational stars and uses information in a star catalog to locate
itself, and its associated vehicle, in space. However, instead of
imaging a navigational star through clear space, a star tracker may
image the navigational star through an atmospheric limb of the
earth. As viewed from space, a star passing behind earth's upper
atmosphere appears to shift upward, i.e., away from the center of
the earth, from its true position due to refraction of the star's
light as the light passes through the atmosphere. The amount of
refraction depends on frequency of the starlight and atmospheric
density.
[0102] A measurement of the refraction of a known star's light near
the horizon can be used to infer a direction, in inertial space,
from the measurement point, toward the portion of the atmosphere
that refracted the light. A star tracker can directly measure this
refraction. Alternatively, a difference in refraction, i.e.,
dispersion, between two different wavelengths, such as red and
blue, of starlight can be measured. This concept is referred to as
stellar horizon atmospheric dispersion ("SHAD"). However, it should
be noted that these two methods are merely different ways of
measuring the same basic phenomenon. The relationship between
refraction and dispersion is well known for air. Using measured
refraction for inferring direction is called stellar horizon
atmospheric refraction ("SHAR"). Embodiments of the present
invention may be used for SHAD- and SHAR-based navigation.
[0103] As noted, passage of starlight 2000 through the earth's
atmosphere bends rays of the starlight inward, as shown
schematically in FIG. 20. Viewed from space, the star's apparent
position 2002 remains on the horizon long after its true position
has "set." A refracted blue ray 2004 observed by the camera 2006
appears to graze the earth 2008 at a height h.sub.a, but actually
grazes the earth 2008 at a slightly lower height h.sub.g. The
actual refraction angle is indicated at 2010. The earth's radius is
indicated in FIG. 20 as r.sub.e.
[0104] The refraction is strongest near the surface of the earth
2008, progressively becoming weaker at progressively higher
altitudes, due to the decreasing density of the atmosphere. For
example, starlight is refracted approximately 330, 150 and 65
arcseconds for grazing heights of 20, 25 and 30 km, respectively.
Lower altitudes, such as about 6 km or 9 km, produce larger
refractive angles, leading to larger signals and higher accuracies.
SHAR is applicable up to about 30.degree. from the horizon and can
be used to provide location updates with accuracies on the order of
.+-.3 meters.
[0105] In effect, the atmosphere acts like a prism, refracting and
dispersing the starlight passing through it. A ray of starlight
passing through the spherical shell of the atmosphere encounters
the gradient in air density, which determines an amount by which
the starlight is bent. Densities of air near the earth's surface
are known to be closely described by an exponential function of
altitude. The amount of refraction depends on frequency of the
starlight. Thus, red light ray 2012 is refracted less than blue
light ray 2004.
[0106] Assuming a spherically symmetric atmosphere, all starlight
refracted by a given amount defines a conical surface 2100
extending into space and having an axis 2102 passing through the
center of the earth in the direction of the star, as schematically
illustrated in FIG. 21. Observation of this particular value of
refraction by a vehicle indicates it is somewhere on the surface of
the cone 2100. By repeating the same type of observation on stars
in different directions, the vehicle can determine its complete
position by essentially solving for intersections of the various
cones.
[0107] However, it is seldom necessary to solve for cone
intersection, because the vehicle typically has sufficiently
accurate information about its position before each measurement to
permit it to use a simpler technique to update its position. At the
time of a measurement, the vehicle typically has a prior estimate
of its position, which is in the vicinity of a small region of the
cone. Because the measurement indicates the vehicle is on the cone,
the most probable position is a point on the cone closest to the
estimated position. Thus, the vehicle can update its position along
a perpendicular line from the estimated vehicle position to the
cone surface.
[0108] This technique provides positional information in only one
dimension. However, similar updates for horizon stars in other
directions throughout an orbit or along another trajectory can
provide a complete update of position and velocity. The star
catalog 1606 (FIGS. 16 and 18) can include data about the
atmospheric limb, in addition to ephemeris data about stars, to
facilitate SHAR- or SHAD-type navigation using an embodiment of
star trackers disclosed herein. It should be noted that SHAR- and
SHAD-type navigation are independent of the GPS and ground-based
tracking systems. Thus, a star tracker that employs SHAR or SHAD
can be autonomous, i.e., independent of any other system.
[0109] Additional information about position determination using
SHAD or SHAR is available in "Satellite Autonomous Navigation with
SHAD," by R. L. White and R. B. Gounley, April, 1987, CSDL-R-1982,
The Charles Stark Draper Laboratory, Inc., 555 Technology Square,
Cambridge, Mass. 02139, which is the assignee of the present
application, the entire contents of which are hereby incorporated
by reference herein.
Artificial Satellites as Navigational Reference Points
[0110] Although star trackers that use navigational stars has been
described, other light-emitting or light-reflecting space objects
can be used for navigation. For example, most artificial satellites
have predictable orbits or other trajectories and can, therefore,
be used instead of, or in addition to, stars for navigation. This
concept was originally proposed by The Charles Stark Draper
Laboratory, Inc. and named Skymark. The star catalog 1606 (FIGS. 16
and 18) can include ephemeris data about artificial satellites to
facilitate Skymark-type navigation using an embodiment of star
trackers disclosed herein. Artificial satellites can also be
sighted through the atmospheric limb, thereby combining Skymark and
SHAR/SHAD techniques. The selectable field of view provided by
embodiments of the present invention enable start trackers to image
even relatively dim objects that are apparently close to very
bright objects.
Methods
[0111] FIG. 22 contains a flowchart illustrating operations of some
embodiments of the present invention. To expose a selectable
portion, less than all, of a field of view of a camera to a scene,
at 2200 a baffle assembly is disposed adjacent the camera, such
that the camera is aimed toward an interior of the baffle assembly.
As noted at 2202, the baffle assembly is configured to define an
aperture whose position on the baffle assembly is electronically
adjustable and such that the aperture defines the selectable
portion, less than all, of the field of view of the camera exposed
to the scene, such as the sky. At 2204, under control of a
processor, the position of the aperture on the baffle assembly is
adjusted, such that the aperture is oriented toward the scene.
[0112] At 2206, a first image is automatically captured by the
camera. Optionally, at 2208, a portion, less than all, of the image
is automatically analyzed, such as to determine a location in space
of the camera. The portion of the image that is analyzed may
correspond to the portion of the camera field of view exposed to
the scene. Analyzing only a portion of the image conserves
resources that would otherwise be required to analyze image
portions that were not exposed to any portion of the scene.
[0113] As noted at 2210, the camera may include several image
sensor arrays, and each image sensor array may include many pixels.
A subset, fewer than all, of the pixels of the sensor arrays may be
read. The subset may correspond to the selectable portion of the
camera field of view exposed to the scene. Reading only a subset of
the pixels conserves resources, such as bandwidth, that would
otherwise be required to read all the pixels in the image sensor
arrays, thereby reducing time required to read relevant pixels.
Generally, the unread pixels were not exposed to any portion of the
scene.
[0114] After adjusting the position of the aperture (2204) and
capturing the first image (2206), at 2212 the position of the
aperture can be further adjusted on the baffle assembly, such that
a different portion of the camera field of view is exposed to the
scene. At 2214, a second image is captured by the camera.
[0115] Optionally, as indicated at 2216, vibration of the camera
may be measured using two orthogonally oriented rate sensors and,
as indicated at 2218, one or more of the captured images may be
analyzed based on the vibration. For example, position of one or
more space objects in the image(s) may be adjusted to compensate
for the vibration. Each image may be adjusted differently,
depending on a measured displacement, acceleration or angular rate
detected by the sensors.
[0116] As indicated at 2220, a location of the camera and,
therefore, a vehicle to which the camera is attached, may be
determined, based at least in part on an analysis of at least a
portion of the first image and, optionally, at least a portion of
the second image.
[0117] As noted, at 2204, the position of the aperture is adjusted.
FIG. 23 contains a flowchart illustrating operations that may be
performed as part of adjusting the aperture, according to some
embodiments of the present invention. As noted at 2300, the baffle
assembly may include a dome that defines an elongated opening (gap)
extending along a longitude of the dome. At 2302, a curtain is
disposed within the opening. The curtain is movable along the
longitude of the dome. The curtain obscures the opening from the
camera field of view, except where the curtain defines the
aperture.
[0118] As shown at 2304, adjusting the position of the aperture may
include rotating the dome about an axis of symmetry of the dome,
such that the opening in the dome is oriented toward the scene. The
rotation is performed under control of a processor. Also under
control of the processor, at 2306 the curtain is moved along the
longitudinal of the dome, such that the aperture is oriented toward
the scene.
[0119] As noted, at 2204, the position of the aperture is adjusted.
FIG. 24 contains a flowchart illustrating operations that may be
performed as part of adjusting the aperture, according to some
embodiments of the present invention. As noted at 2400, the baffle
assembly may include a dome that includes elements. Transparency of
each element is electronically controllable.
[0120] As shown at 2402, adjusting the position of the aperture may
include setting transparency of at least a selected one of the
elements, such that the selectable portion of the field of view of
the camera is exposed to the scene through at least one transparent
element, and a remaining portion of the field of view of the camera
is obscured from the scene by at least one non-transparent element.
The element transparencies are set under control of a
processor.
[0121] Optionally, at 2404, adjusting the position of the aperture
on the baffle assembly may include setting transparency of the
selected element to adjust size of the aperture. For example, a
group of adjacent elements may be made transparent, and surrounding
elements may be made non-transparent. The size of the aperture is
determined by the number of adjacent transparent elements, and of
course size of each element. The element transparencies are set
under control of a processor.
[0122] As noted, at 2204, the position of the aperture is adjusted.
FIG. 25 contains a flowchart illustrating operations that may be
performed as part of adjusting the aperture, according to some
embodiments of the present invention. At 2500, the aperture is
adjusted such that the selectable portion of the camera field of
view includes a portion of the scene expected to include a space
object having a predictable location. As noted at 2502, the space
object may be an astronomical object, such as a star, a planet or a
natural satellite, or an artificial satellite.
[0123] At 2504, an image is captured with the camera, and at 2506 a
location of the camera is automatically determined, based at least
in part on information about the space object and an analysis of at
least a portion of the image. As noted at 2508, determining the
location of the camera may include determining the location based
at least in part on dispersion or refraction of light from the
space object through earth's atmospheric limb, such as using a SHAD
or SHAR technique.
Implementation Details
[0124] Some star trackers, according to the present disclosure, can
provide navigational accuracy approximately equivalent to the GPS,
i.e., an error of approximately .+-.3 meters. Earth's circumference
is approximately 40,075 km, and it has 360.degree. of
circumference. Equation (1) shows that approximately 0.097
arcseconds of sighting accuracy is needed to achieve .+-.3 meters
in positional accuracy.
(3/40075000)*360.degree.=0.097 arcseconds (1)
[0125] System accuracy is determined by the field of view subtended
by each pixel in the camera's image sensor arrays 912-918, etc.,
known as an instantaneous field of view (iFOV). Using standard
centroiding techniques, sub-pixel accuracy can be achieved. In one
embodiment, the objective lens 902 has a 120.degree. (2.09 rad)
field of view, and each pixel in the camera's image sensor arrays
is about 8.5 .mu.m across and has an iFOV of 0.2 mrad (40
arcseconds). The lens has an F number of about 1.7. Equation (2)
shows that approximately 10,472 pixels are necessary to diagonally
cover a 120.degree. (camera) field of view.
(2.09 rad/0.2 mrad).apprxeq.10,472 pixels (2)
[0126] Assuming each image sensor array 912-918, etc. has an aspect
ratio of 16:9 and the image sensor arrays 912-918, etc. are
conceptually concatenated to form a rectangular image area (also
having a 16:9 aspect ratio), a corner-to-corner diagonal of the
concatenated image area has an angle of 29.36.degree.. Equations
(3), (4) and (5) show the number of horizontal pixels, the number
of vertical pixels and the total number of pixels in the
concatenated image area.
10472*cos(29.36.degree.)=9127 pixels(horizontal) (3)
10472*sin(29.36.degree.)=5134 pixels(vertical) (4)
9127*5134=46,858,656 pixels(total) (5)
[0127] Thus, the total number of pixels in all the image sensor
arrays is approximately 50 million.
[0128] Sighting accuracy is determined by brightness of the star
being observed, compared to noise of the camera, i.e., a
signal-to-noise ratio (SNR). The SNR limits an extent to which the
centroid of the star can be accurately determined and sets a design
parameter for the celestial sighting system. Calculations have
shown a 2.5 cm aperture 120 meets the 0.1 arcsecond accuracy needed
to achieve .+-.3 meter positional accuracy, as summarized in Table
1.
TABLE-US-00001 TABLE 1 Sighting accuracy calculation assumptions
Star magnitude 3 Effective aperture diameter 2.5 cm Quantum
efficiency (pixel) 0.75 Dark current noise 2.12 e/exposure Read
noise 5 e Limb flux noise 5 e/pixel/exposure Integration time 0.01
sec./exposure Signal 3,949 photons/exposure Total noise 63.27
e/exposure SNR per exposure 62.4 Sighting time 1 sec. Number of
exposures 100/sec. SNR of sighting 624 Number of pixels (diagonal)
10,472 Number of pixels (total, all sensors) 52 million (16:9
aspect ratio) Pixel size 8.50 .mu.m Region of interest 300 pixels
Region of interest field of view 3.44.degree. Data rate 9
Mpixels/sec. Sensor field of view 120.degree. Pixel instantaneous
field of view 2.00E-04 rad./pixel Pixel subtense (DAS) 41.25
arcseconds/pixel Wavelength 1.00E-04 cm (1,000 nm) Sighting
accuracy 0.999 arcsecond
[0129] In some cases, such as where the star tracker is attached to
an artificial satellite or other space vehicle, optics and
electronics of the star tracker may require thermal stabilization
to ensure dimensional stability necessary to meet the 0.1 arcsecond
accuracy specification. Space-based embodiments should include a
thermal design that passes dissipated heat through the camera to
the vehicle in a consistent flow. Airborne and ground-based system,
such as jeep-mounted or soldier-mounted navigation systems, may
require forced airflow to avoid undesirable thermal gradients.
[0130] Atmospheric turbulence can have a significant effect on
airborne and ground-based sightings. Accurate weather updates may
be used to by the controller to compensate for these effects.
Optionally or alternatively, averaging multiple sightings taken in
a relatively short period of time may compensate for atmospheric
turbulence. A frame rate of about 100 images/sec. facilitates
taking a sufficient number of sightings in a sufficiently short
period of time.
[0131] Sighting during daytime presents additional atmospheric
issues. Atmospheric scattering of light causes a high background
level of illumination, through which a star or satellite sighting
must be taken. However, some stars and artificial satellites are
bright enough to be imaged against this background sky
brightness.
[0132] The system may be initialized by executing a rapid, low
accuracy scan to perform a lost-in-space attitude determination.
This can be accomplished by sweeping the baffle through a large
angle, thereby capturing a large field of view of the sky,
containing sufficient navigational fiduciary markers to support the
lost-in-space algorithm. A series of images may be captured as the
baffle is swept. Alternatively, one (relatively long) image may be
captured while the baffle is swept. Orientation information
obtained from the initial scan needs to be only accurate enough so
the baffle can be then be directed toward a star on the horizon, so
a (more accurate) SHAR-based analysis can be performed. Optionally,
the star tracker includes a coarse sun sensor, so the star tracker
can avoid imaging the sun, thereby speeding the initial scan.
Optionally, if another navigational system, such as an inertial
navigation system (INS) or GPS, is available, it can be used to
obtain the initial attitude.
Other Applications
[0133] A star tracker, as describe herein, may be used in submarine
and unmanned undersea systems. In one embodiment, a star tracker is
mounted atop a mast extending from a submerged vehicle to above the
water's surface. The controller uses one or more images taken by
the camera to ascertain a direction of the sun, moon or other
bright object and to direct the aperture toward a portion of the
sky not in the direction of the bright object and then capture one
or more images of navigation stars, artificial satellites,
land-based light beacons or other fiduciary markers. After
analyzing the first one or more such images, the controller
calculates an approximate location and orientation of the star
tracker and directs the aperture toward one or more other expected
navigational fiduciary markers and captures one or more additional
images. The angular rate sensors are used to measure ship motion,
so the controller can account for this motion in its position
calculations. It should be noted that no radar or other radio
frequency transmission is involved, thereby frustrating detection
by an adversary. Using a wide field of view, such as by making
many, most or all of the electro-optic pixels of the dome
transparent, or by sweeping the mechanical baffle across large
portions of the sky, the star tracker can capture an image of much
of the sky, such as at night, and calculate a location using many
navigational fiduciary markers.
[0134] A star tracker, as described herein, may be used in parallel
with another navigation system, such as a GPS, as a backup, in case
an on-board GPS receiver fails or the GPS is compromised. The star
tracker may be used to verify a GPS-determined position and take
over if the verification fails.
[0135] While the invention is described through the above-described
exemplary embodiments, modifications to, and variations of, the
illustrated embodiments may be made without departing from the
inventive concepts disclosed herein. Furthermore, disclosed
aspects, or portions of these aspects, may be combined in ways not
listed above and/or not explicitly claimed. Accordingly, the
invention should not be viewed as being limited to the disclosed
embodiments.
[0136] Although aspects of embodiments may have been described with
reference to flowcharts and/or block diagrams, functions,
operations, decisions, etc. of all or a portion of each block, or a
combination of blocks, may be combined, separated into separate
operations or performed in other orders. All or a portion of each
block, or a combination of blocks, may be implemented as computer
program instructions (such as software), hardware (such as
combinatorial logic, Application Specific Integrated Circuits
(ASICs), Field-Programmable Gate Arrays (FPGAs) or other hardware),
firmware or combinations thereof.
[0137] Some embodiments have been described as including a
processor-driven controller. These and other embodiments may be
implemented by a processor executing, or controlled by,
instructions stored in a memory to perform functions described
herein. The memory may be random access memory (RAM), read-only
memory (ROM), flash memory or any other memory, or combination
thereof, suitable for storing control software or other
instructions and data. Instructions defining the functions of the
present invention may be delivered to a processor in many forms,
including, but not limited to, information permanently stored on
tangible non-writable storage media (e.g., read-only memory devices
within a computer, such as ROM, or devices readable by a computer
I/O attachment, such as CD-ROM or DVD disks), information alterably
stored on tangible writable storage media (e.g., floppy disks,
removable flash memory and hard drives) or information conveyed to
a computer through a communication medium, including wired or
wireless computer networks.
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